Reduction in both size and power consumption of integrated circuits has led to the proliferation of wireless technology. For example, there is a wide variety of devices using low-power wireless circuits; including tablets; smartphones; cell phones; laptop computers; MP3 players; telephony headsets; headphones; routers; gaming controllers; mobile internet adaptors; wireless sensors; tire pressure sensor monitors; wearable sensors that communicate with tablets, PCs, and/or smartphones; devices for monitoring livestock; medical devices; human body monitoring devices; toys; etc. Each of these devices requires a standalone power supply to operate. Typically, power supplies for these devices are electrical batteries, often replaceable batteries.
Other wireless technologies of significant interest are wireless sensors and wireless sensor networks. In such networks, wireless sensors are distributed throughout a particular environment to form an ad hoc network that relays measurement data to a central hub. Particular environments include, for example, an automobile, an aircraft, a factory, or a building. A wireless sensor network may include several to tens of thousands of wireless sensor “nodes” that operate using multi-hop transmissions over distances. Each wireless node will generally include a sensor, wireless electronics, and a power source. These wireless sensor networks can be used to create an intelligent environment responding to environmental conditions.
A wireless sensor node, like the other wireless devices mentioned above, requires standalone electrical power to operate the electronics of that node. Conventional batteries, such as lithium-ion batteries, zinc-air batteries, lithium batteries, alkaline batteries, nickel-metal-hydride batteries, and nickel-cadmium batteries, could be used. However, it may be advantageous for wireless sensor nodes to function beyond the typical lifetime of such batteries. In addition, battery replacement can be burdensome, particularly in larger networks with many nodes.
Alternative standalone power supplies rely on scavenging (or “harvesting”) energy from the ambient environment. For example, if a power-driven device is exposed to sufficient light, a suitable alternative standalone power supply may include photoelectric or solar cells. Alternatively, if the power-driven device is exposed to sufficient air movement, a suitable alternative standalone power supply may include a turbine or micro-turbine for harvesting power from the moving air. Other alternative standalone power supplies could also be based on temperature fluctuations, pressure fluctuations, or other environmental influences.
Some environments do not include sufficient amounts of light, air movement, temperature fluctuation, and/or pressure variation to power particular devices. Under such environments, the device may nevertheless be subjected to fairly predictable and/or constant vibrations, e.g., emanating from a structural support, which can be in the form of either a vibration at a constant frequency, or an impulse vibration containing a multitude of frequencies. In such cases, a scavenger (or harvester) that essentially converts movement (e.g., vibrational energy) into electrical energy can be used.
One particular type of vibrational energy harvester utilizes resonant beams that incorporate a piezoelectric material that generates electrical charge when strained during resonance of the beams caused by ambient vibrations (driving forces).
Flatness and resonant frequency of microelectromechanical (“MEMS”) cantilever structures used for piezoelectric energy harvesting are important for their efficient operation. Typical MEMS cantilever structures used in energy harvesting devices are a laminate of multiple layers, each with a specific function (strength, resonant frequency tuning, conduction, piezoelectric harvesting). The residual stress of each layer must be strictly controlled to produce a flat structure. The buckling or cupping in the cantilever due to poor residual stress control can impact the full width half maximum of the resonant response, maximum power, quality factor (damping coefficient), and robustness of the device. One way reported in the literature to mitigate some of these issues is to angle or taper the cantilever in the plane parallel to the substrate. However, this does not relieve all of the stress “hotspots” observed. Further structuring the cantilever sidewall shape to follow the stress contours of the laminate allows for another degree of freedom in engineering the flatness of the cantilever.
Moreover, in some instances, efficient operation of MEMS cantilever structures for piezoelectric energy harvesting also requires precise tuning of the peak resonant response. Changing the taper angle of the cantilever in the plane parallel to the substrate impacts the resonant frequency by reducing the stiffness of the beam. However, this approach can also lead to poor clamping of the end mass and subsequent robustness issues.
The present invention is directed to overcoming these and other deficiencies in the art.